That looks to me like one of the fireweed aka willowherb - epilobium.

Maybe try digging rather than pulling, you may be leaving some perennial roots in the ground. Also, if it looks like above photo, you've waited too long to deadhead, those fluffy little bits are the seeds and its just about impossible to weed out the plants without dispersing the seeds at this point. Never let a plant you want to remove go to seed, you defeat your purpose.

Work a bit harder at the roots, weed any seedlings out much earlier. If its set seed a year or two, you're going to be finding the seedling for a few seasons but they aren't hard to control if you give them your attention earlier.

I don't know what herbicide you tried but there's no reason it should not be effective before flowers and seeds have formed - but easily recognizable, you can control it manually by paying attention in Spring....you never want to let it go until you see the fluffy bits, my heart sinks if I ever find them, almost have to take a bag into the garden to carry out the weeded plant without sowing the seed as I'm leaving....